Electron-optical system and method



Sept. 1 E. GUNDERT ETAL 2,954,499

ELECTRON-OPTICAL SYSTEM AND METHOD 2 Sheets-Sheet 1 Filed March 26, 1958 lm/emars Eb erficzrn Gander-f 6ew1 V/brans Sept. 27, 1960 E. GUNDERT ETAL 2,954,499

ELECTRON-OPTICAL SYSTEM AND METHOD Filed March 26, 1958 2 Sheets-Sheet 2 IO; //ll\6 H4] 5 mafia FIG 4 INVENTORS Eberhard Gunderr&

Gerwig Vibrans 1 I v BY 91.) '4

ATTORNEY 2,954,499 Patented Sept. 27, 1960 ELECTRON-OPTICAL SYSTEM AND mrnon Eberhard Gundert, Ulm (Danube), and Gerwig Vihrans, Braunschweig, Germany, assignors to Telefnnken G.m.b.H., Berlin, Germany Filed Mar. 26, 1958, Ser. No. 724,147

Claims priority, application Germany Mar. 27, 1957 5 Claims. (Cl. 315-17) The invention relates to an electron-optical system for cathode ray tubes employing after-acceleration of the deflected beam.

It has been known to after-accelcrate the electron beam between the deflecting device nearest the screen and the screen itself. This acceleration is obtained by causing the electrical potential along the envelope to increase either suddenly or gradually. For this purpose the potential along central axis Z of a symmetrical envelope is usually itself symmetrical about the axis of the envelope. It has been found that a potential with increasing steepness acts upon the beam in a manner which may be described best as causing a converging effect, while a potential with decreasing steepness acts divergently. Most of the systems heretofore known have an overall converging effect. Consequently, the beam in these known systems is bent toward the above mentioned axis Z which, of course, is also the axis of the electron-optical system. This bending of the beam decreases its deflection so that the sensitivity is likewise decreased. In order to avoid such influence of the last mentioned bending effect, certain proportions in the system have to be.observed. More specifically the ratio of the after-acceleration voltage to the anode voltage should have a value of 1.0, or less.

It has been attempted to reproduce an enlarged inverse intermediate image on the screen. by employing strongly converging after-acceleration, and thus. to obtain a larger deflection sensitivity by means of an after-acceleration lens than in the absence thereof. However, in this case the cross sectional area of the electron beam is increased to a great extent; this increase is to be understood as viewed from the intermediate image towards the cathode. In view of this last mentioned effect, the deflecting means of the system has to be positioned in the same axial zone of the beam as that in which the intermediate image appears, to avoid a great increase in cross section of to beam within the effective range of the deflecting means;

This result can be obtained by the use of electrostatic or magnetic deflecting systems. However, such deflecting systems are complex, and they cause distortions of the image. Consequently, such magnetic or electrostatic systems have not been used in practice for the mentioned purpose.

It has been known to provide an electron lens between the deflecting means and the screen, said lens acting as a toric lens or a cylindrical lens. The provision of such lens increases the deflection sensitivity and the resolving power of a cathode ray tube. However, in these systems diverging cylindrical lenses have been provided heretofore to increase the deflection sensitivity.

It is an object of the present invention to provide new and efficient means to increase the deflection-sensitivity of cathode ray tubes.

It is another object of this invention to provide in an electron-optical system a method and means for afteraccelerating an electron beam within a cathode ray tube, wherein this after-acceleration takes place between the deflection means closest to the screen and the screen itself. According to the invention, convergent after-acceleration occurs primarily only in the plane of deflection caused by the last mentioned deflecting means. Additionally, an intermediate image is produced between the screen and the deflection means which is closest to the cathode, whereby this intermediate image appears reversed on the screen only in one of the deflection planes. The main image on the screen is derived from said intermediate image by after-acceleration.

Still further objects and the entire scope of applicability of the present invention will become appatfint from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific example, while indicating a preferred embodiment of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

In the drawings:

Fig. 1 of the drawings is a diagram of potential gradients along the center axis of a cathode ray tube with after-acceleration.

Fig. 2 illustrates schematically a side view of a priorart electron-optical system.

Fig. 3a illustrates schematically a side view of an electron-optical system according to the invention.

Fig. 3b illustrates a top plan view of the system shown in Fig. 3a.

Fig. 4 illustrates schematically a side view of a cathode ray tube incorporating an electron-optical system according to the present invention.

In the prior-art diagram of Fig. l, y denotes the potential along a center axis Z of an axially symmetrical electronoptical system showing that the after-acceleration potential gradient follows the curve a or b. According to the laws of electronoptics the portion a of the curve a is bent upwardly, thereby causing a converging effect in the electron beam. The portion a" of the curve a is bent downwardly, thus causing a diverging effect in the beam. The curve b is bent upwardly over its entire length, causing a converging effect on the beam over the entire length of the field. Since the field range of higher voltages is passed by electrons of higher velocity, the action of the converging field in case of potential gradients according to curve a is always higher. Due to higher velocity, the total effect of a voltage according to curve a is also a converging effect, as the converging portion a prevails. As mentioned in the foregoing, the sensitivity of the system is decreased when the electron beam is bent towards the center axis Z under the influence of afteracceleration. The principle and problems of a converging after-acceleration field will now be explained, with reference to the schematic prior-art illustration of Fig. 2, in which lenses are indicated by symbols as conventionally used in optics, although the lenses referred to hereinafter are always to be understood as electronic lenses. of which various kinds have been known in the art and may be used in the embodiments according to this invention.

In Fig. 2, a cathode 1 emits electrons l intersecting at a point 2 due to the concavity of the emitting surface of this cathode. The system includes a main focusing lens 3. A deflecting field 4 is set up, said field acting in the plane of the drawing. Thereafter, follows an afteracceleration converging lens 5, and then a screen 6. preferably a fluorescent screen conventionally used in cathode ray tubes. The after-acceleration lens 5 produces an image of the center point 7 of deflection at 8, i.e., at the point where the beam intersects the axis Z. This point 8 ordinarily has such a distance from the screen 6 that the overall or total deflection 20 of the beam is sufli- -Theimageof point 2 in plane 21 has to be sutficiently 5 small in order to obtain a sharply focused spot 9 on the screen 6. I This can be obtained by determining a minimum beam angle 8 at the screen and preventing the diameter of the beam in the after-acceleration lens 5 from becoming too small. The main converging lens 5 has to 10 be positioned as close as possible to the plane 21 in order to avoid an unnecessarily large diameter of the beam within this lens 5. p

In view of this requirement, the deflection. means for I the two deflection planes should not be arranged one behind the other on the axis Z. This difficulty is overcome according to the invention'by producing an intermediate image in only one of the planes of deflection, for example, by means of a cylindrical lens 13 disposed between the two deflection means 14 and 11, 'Whereby the 0 conventional deflecting systems with deflecting means arranged on the tube axis can be employed. Under these conditions, however, the after-acceleration field cannot be symmetrical, i.e., this field has to be asymmetrical" at least in its converging field portion. The basic law for the systems described'is the Laplace-equation:

en. i 2

due to the potential gradient it 32 along the axis. Thus i 4 3x and are determined by virtue of the second derivative of the potential along the center axis Z. In'toric fields and only their sum is given by according to the Laplace-equation. Consequently, the after-acceleration field component acting in the plane of the deflecting means nearest the screen mustbe sufficiently diflerent from the field component acting perpendicular to that plane. Thus, the field can be rendered elfective 65 as a converging field in one plane and be only slightly converging or even divergent in the other plane.

Keeping this in mind, the embodiments of the invention shown in Figs. 3a and 3b can be more readily understood, in which the same reference numeral as in Fig. 2 are used for like parts. An acceleration field can be produced for example by a helical coating at the inner surface of the bulb. Such a helical coating is schematically shown in the Figs. 3a and 3b by the dotted lines 17. 10 denotes an after-acceleration field component having a curve particularly useful for the invention. This component 10 of the after-acceleration field is convergent only in the plane of the drawingof Fig. 3a, while a component 15 of this field is effective divergently only in the plane of the drawing of Fig. 3b. The field component 111' is, for example, cooperative witht-the deflection plates 11 (Fig. 3a) which are the deflection means nearest the screen, while the component 15 is, for example, cooperative with the deflection plates 14 (Fig. 3b) which are the deflection means disposed nearest the cathode. The beam is bent towards the axis Z in the deflection plane of Fig. 3a only if the after-acceleration is sufficiently great. The intermediate image of the electron spot now appears at point. 12 in the plane of Fig. 3a. The image appears on line 12' in'Fig. 3b. A lens 13 provided for proper adjustment of the image 12,12f is positioned between the deflection plates 14, 14' and the plates 11, 11'. This lens 13 is convergent substantially only in the plane of Fig. 3a, which is the deflection plane of the plates 11, 11. Thus the lens 13 is suitably made cylindrical. The deflection of the beam due to the plates 14, 14' is substantially not changed by this lens 13 within the plane of Fig. 3b, or maybe increased by the field component 15 due to the divergent action of the after-acceleration field. The presentation of the lens 13 by dashed lines in Fig. 3b shall indicate it is without influence in the drawing plane of this Fig. 3b.

It is recommended to have the refraction power of the lens 13 depended upon the deflection of the plates 14, 14' in the plane of drawing of Fig. 3a rather than constant, whereby the variation of the refraction power within this plane is selected so that the distortions of the spot due to the deflections of the plates 14, 14' are as small as possible. The main converging lens 16 may be axially symmetrical, cylindrical or t-oric.

Figure 4 is a schematic representation of a cathode ray tube incorporating an electron-optical system according to the present invention. The cathode 101 emits an electron beam passing through the main converging lens 116 which may, as stated above, be axially symmetrical, cylindrical or toric, whereafter the beam is deflected by a set of plates only one of which is shown at 114. The beam then passes through a lens 113 which is shown as being composed of two successive slotted field stops, after which the beam passes between two deflecting plates 111 and 111' to reach the screen 106. The helical coating on the inside of the tube is shown at 117.

We claim:

1. An electron-optical system for a cathode ray tube having a cathode for emitting a beam and a screen, said system comprising: first deflection means nearest the cathode; second deflection means nearest the screen, said first and second deflection means being arranged one behind the other symmetrically about the axis of the beam and respectively deflecting the beam transversely of the axis in mutually perpendicular planes; after accelerating means arranged between said second deflection means and the screen for providing a field having a gradient along the beam axis substantially only in the deflection plane of said second deflection means; and lens means arranged between said first and second deflection means for producing an intermediate image only in said de flection plane of said second deflection means and substantially intermediate said lens means and said screen, said after accelerating means converging the beam to cross said beam axis and to reproduce said image on said screen. r

2. An electron-optical system according to claim 1, wherein the refractive power of the lens means in the deflection plane of the first deflection means is. designed to 'Vary as a function of the angle of deflection such that the spot distortions caused during deflection by the first deflection means are minimized.

3. An electron-optical system according to claim 1, wherein a main collecting lens is interposed between the References Cited in the file of this patent UNITED STATES PATENTS Ressler Apr. 19, 1938 Hogan Aug. 27, 1940 Harrison Oct. 30, 1951 Biggs Nov. 18, 1958 FOREIGN PATENTS Great Britain Oct. 18, 1937 Great Britain May 24, 1943 

